A Colorado programme that offers free birth control to teenagers has dramatically reduced the rate of teenage pregnancy. But the nature of the scheme's funding - a large anonymous donation - leaves it unclear whether it could work on a broader scale.

Dianzu Mosqueda Salinas is a young woman working at a family planning centre in the Colorado town of Boulder.

In 2010, she had walked into the very same centre as a nervous teenager, curious about birth control options. . . .